1. Field of the Invention
This invention relates to improved devices and methods for liquid overflow protection, and more particularly to fail-safe lockout circuits and methods of using same that are of particular use in automatic washing machines and in other applications in which a tub is filled by a liquid passing through a controllable valve.
2. Description of the Prior Art
FIGS. 1 and 2 show prior implementations of an electromechanical (E/M) system and an electronic system, respectively, for measuring and controlling the water level in a washing machine.
In the prior art E/M system of FIG. 1, a pressure switch 100 controls the desired water level setting (e.g., high, medium, or low) and applies AC line power to a temperature select switch 102. This switch is used to direct the AC line power to the appropriate water valve solenoids 104 or 106 depending upon whether a hot, warm, or cold fill is selected. Whenever the actual water level is below the desired level, pressure switch 100 remains closed and the water valve solenoids are energized. When the actual water level reaches or exceeds the desired water level, pressure switch 100 opens and power is removed from the water valves.
The failure modes that can cause an overfill condition in this type of system are primarily associated with pressure switch 100. These primary failure modes are:
(A1) Failure of the pressure switch to respond to the increasing water level; and PA1 (A2) Failure in the pressure tube connected to the pressure switch. PA1 (B1) Failure of the electronic pressure sensor to monitor the water level; PA1 (B2) Failure of the interface circuit to properly convert the pressure sensor signal; PA1 (B3) Failure of the microprocessor to properly execute the program; PA1 (B4) Failure of the water valve driver circuits to properly respond to the microprocessor input signals; and PA1 (B5) Failure of one or both of the water valves to turn off when AC power is removed.
In the prior art electronic system illustrated in FIG. 2, an electronic pressure sensor 110 is used to measure the actual water level. An interface circuit 112 converts this measurement to a signal suitable for presentation to an input of microprocessor 114, which can thus monitor the water level at all times during the filling operation. The desired water level and temperature settings are logic inputs 116 and 118 to microprocessor 114. Using these logic inputs, microprocessor 114 controls the signals to valve driver circuits 120, 122 while also monitoring the signal from interface circuit 112 corresponding to the water level sensed by pressure sensor 110. When this signal corresponds to the desired water level setting, microprocessor 114 terminates the valve driver input signals to valve drivers 120, 122, causing the water valves 124, 126 to be turned off.
Since in this implementation a number of failure modes can occur that can cause an uncontrolled fill condition, a second E/M pressure switch 128 is normally added to the system to control the AC power to the water valve driver circuits 120, 122. Pressure switch 128 is normally closed and opens only if the water level becomes unusually high. This unusually high water level is set above the active range of electronic pressure sensor 110 so as not to interfere with normal machine operation. If the extra "overfill" pressure switch were not used, some of the additional failure modes that could cause an overflow condition beyond those listed for the E/M system are:
The addition of the "overfill" pressure switch does reduce the potential overflow failure modes, but does so at a cost penalty. It also requires an additional connection via a pressure tube to the pressure switch, and additional labor is required to mount it.
Known prior art devices and methods that either indicate or correct the occurrence of abnormal operation in washing machines are known, but these known devices and methods are each subject to at least one or more of the failure modes listed above depending upon the class of system to which they belong. For example, U.S. Pat. No. 4,195,500 (Tobita et al.) discloses a timer responsive to a pressure sensor for sensing fill level in a microprocessor-controlled washer. Water from a hose proceeds into a basket, and the change in weight is sensed by a weight sensor. By comparing the measured rate of change in weight with previously stored values, it can be determined whether a predetermined change of weight is attained after a prescribed time period during the water filling operation. If abnormal operation is determined, a stop signal is supplied to a drive circuit, which de-energizes circuits to stop the abnormal operation. A pressure switch also produces a signal when the water level in the tub reaches a predetermined level. A CPU responds to this signal. If a longer time is required than expected, it is presumed that the water did not reach the predetermined fill level, and operation is stopped. If a non-full signal level is detected, the feed water is continued. In this device, all control and timing is provided by the microprocessor, and a failure of the pressure switch cannot be detected.
In U.S. Pat. No. 5,305,485 (Getz et al.), a washer is disclosed that has an electroacoustic transducer provided on an openable lid. A microprocessor directs the transducer to send a pulse to the surface of a load of clothes, which reflects a pulse back to the transducer. From this signal, the microprocessor determines the distance to the surface of the clothes load and an optimum liquid level, and thus outputs a signal to control a pair of mixing valves. The water level during the filling operation is monitored by a pressure sensor, which outputs a signal having a frequency indicative of pressure. The microprocessor monitors this signal to determine the water level, and sends out a signal to close the valves when the water level sensed by the pressure sensor corresponds to the required fill level. The operation of the pressure sensors and the microprocessor is not monitored, and thus, failure of either could cause overfilling of the tub.
U.S. Pat. No. 4,611,295 (Fowler) is directed to a supervisor control system for appliances employing a conventional microprocessor. A comparator remains latched as long as signals of a proper frequency are generated by the microprocessor and supplied from an output pin of the microprocessor. If the pulses terminate, a capacitor being discharged by a transistor stops being discharged, which eventually causes a comparator to change state, causing a relay to drop out. If the microprocessor "scrambles" and produces pulses at a rapid rate, a low pass filter prevents the transistor from turning on, allowing the capacitor to charge up and the relay to turn off as though the pulses had terminated. Thus, this devices provides sensing of the "sanity" of the microprocessor, but does not provide fail-safe monitoring of the sensor.
U.S. Pat. No. 4,201,240 (Case) discloses pulsed signals from a sensor that service a counter to shut off water flow. The frequency of the pulsed signals varies in proportion to the changing water level in the tub. This frequency is counted by counter circuitry and sensed by a comparator. Upon sensing certain predetermined values, a control signal is sent to a controller, which can cause a solenoid to shut off the flow of water into the tub. Termination of water flow occurs only after counters reach a predetermined value, and no devices or methods are shown to independently ensure the proper operation of the controller or of the sensing circuits.
It would therefore be desirable to improve the failure mode sensing of washing machines and the like, while at the same time eliminating the need for an additional "overfill" pressure switch. It would also be highly advantageous if the improved failure mode sensing ensured that overfill did not occur because of a failure of either a sensor, a microprocessor, or of both the sensor and the microprocessor.